In U.S. Pat. No. 4,315,100, Haslbeck, there is shown an electrical box with an electrical device attachment structure having a wedging thread engagement means which is employed in the present invention. As shown in that patent, the electrical box is molded of a thermoplastic or thermosetting plastic and a rectangular protrusion is formed at each end on the outside of the box. A downwardly opening, generally triangular cavity within the protrusion may have partial threads along one wall. A triangular cam member fits within the cavity and has partial threads along the side facing the threaded wall. A screw of suitable size can easily be inserted by pushing the screw between the partially threaded regions and then, as the screw is rotated, the threads on the screw engage the threads on the cam member and the wall, forming a secure mechanical engagement. Any axial force tending to extract the screw pulls the sloping surface of the cam member against the similarly sloping surface of the cavity, increasing the compressive force of the partial threads against the screw and preventing its removal.
While this structure functions quite well, there were some practical problems with the arrangement as shown in the Haslbeck patent including the fact that the box must have molded knockouts to provide entry openings for cables, and that the inner surface of the box must be clear between the knockouts and the open side of the box so that the molded box can be ejected from the mold during production. Partly for molding reasons, the effective, usable interior volume of the box was reduced by about 13% or more, dependent upon box depth, a serious loss of wire fill capacity in a box which must receive a switch, outlet or other electrical device and then accommodate wires as well. The loss of wire capacity detrimentally affects the box rating since the National Electrical Code requires that the box have a certain cubic inch capacity in order to receive wires of specific sizes. Simply rotating the original design 90.degree. would create an unacceptable protrusion on each end of the box. The original box thus had limited utility.
An improved arrangement positioned the screw receiving "pocket" at the front end of a concave, outwardly facing channel at the end of the box, the pocket being rotated so that the arcuate surface on the movable cam member faced generally toward the geometric center of the box. This improvement made the box easier to mold and maximized the interior volume of the box and also allowed access knockouts to be molded into the box at almost any desired location.
It developed, however, that when a screw is inserted into the pocket, and before the screw is rotated to cause thread engagement, the end of the screw sometimes abuts the cam member, giving the feeling that there is an obstruction, even though the problem is only one of misalignment. That abutment often induces the user to push harder, causing the movable cam member to be forced out of the pocket and lost.